High-frequency radiation, such as x-rays, is widely used as diagnostic tools for human diseases and injuries. Radiation-sensitive substrates, such as x-ray film, are exposed to radiation that has passed through portions of the human body thereby forming images of internal structures in the body that have differentially absorbed the radiation. In digital radiographic systems, radiation-sensitive materials coated over a substrate and exposed to radiation form a charge pattern in the coated substrate that can form an image when read with electronic circuits.
Digital radiography can provide advantages in medical or other diagnostic work. For example, digital radiographs are reusable and do not require chemical development, thereby decreasing response time and costs. Digital radiography can also be more sensitive to radiation so that radiation exposure to human subjects is reduced. Despite these advantages, digital radiography is expensive and can suffer from electronic noise that reduces the accuracy of the read charge pattern, reducing its diagnostic value.
The radiation-sensitive substrates in digital radiographic systems form flat-panel detectors. In one type of system, substrates coated with photo-stimulable phosphors are exposed to radiation. The photo-stimulable phosphors are exposed to light to produce a signal whose strength corresponds to the amount of radiation exposure incident on the phosphors. After exposure, the plates are placed in a reader that stimulates the substrate with light, for example using a scanning laser, to retrieve the signal over the area of the substrate to produce an image.
In another type of flat-panel detector, a layer of scintillating material (e.g. cesium iodide or gadolinium oxysulfide) coated over a substrate responds to x-ray radiation by emitting photons in proportion to the quantity of incident radiation. The photons are then detected by amorphous silicon photo-diodes to produce a current that is electronically detected. The photo-diodes are distributed over the area of the substrate to provide multiple signals corresponding to pixels forming an image of the x-ray radiation that is used for diagnosis by a radiologist.
Yet another type of flat-panel detector uses a layer of radiation-sensitive material (e.g. amorphous selenium) coated over a substrate between electrodes that responds to x-ray radiation by forming a charge in proportion to the quantity of incident radiation. The charge forms a pattern corresponding to the incident radiation and is detected by electrodes patterned over the substrate that are connected to electronic circuits. Essentially, an array of capacitors are charged in a pattern corresponding to the radiation pattern and the capacitor charges are read with the electronic circuits to form pixel values forming an image of the x-ray radiation that is used for diagnosis by a radiologist.
It is preferable that any electronic signal produced from a digital radiographic exposure be read accurately and quickly. For large substrates, it is difficult to transfer an electronic signal to circuits separate from the radiation-sensitive substrate at high speed and without adding electronic noise, particularly for the signal-measuring circuitry. Furthermore, it is preferable that any substrate have as large a radiation-sensitive area as possible to provide as high-resolution a signal as possible, for example having as many pixels per unit area as possible.
U.S. Pat. No. 5,381,014 describes a large area x-ray image capture element fabricated by juxtaposing a plurality of discrete array modules in an assembly over the top surface of a base plate, such that each module is disposed adjacent at least one other module to form a two-dimensional mosaic of modules. Each of the discrete modules includes a plurality of thin-form transistors arrayed adjacent the top surface of a dielectric substrate wherein at least one precision-ground edge forms a precise abutment with a precision-ground edge of another substrate. A continuous radiation detecting layer is disposed over the plurality of juxtaposed modules and produces a latent radiographic image in the form of electrical charges. Such a method reduces or totally voids the non-radiation-detecting areas created at the borders between the array modules. However, such a design employs thin-film electronic devices that are known to have lower performance than crystalline semiconductor electronic devices. Furthermore, the assembly of the described structure is problematic.
The location of crystalline integrated circuits over a substrate such as a printed-circuit board is known in the prior art, for example using surface-mount integrated-circuit components, such as ball-grid arrays, multi-chip modules, and flip-chips, as well as soldering the components to the printed-circuit board. A variety of packaging and placement techniques, both manual and automated, are known for assembling electronically active substrates. Such integrated-circuit substrate structures, however, can interfere with the ability of a radiation-sensitive substrate to provide a high-resolution, low-noise, image of radiation incident on the substrate.
There is a need, therefore, for an alternative substrate design for providing a high-resolution, low-noise, image of patterned radiation incident on the substrate.